(1) Field of the Invention
The present invention relates to a demodulation apparatus using the OFDM (Orthogonal Frequency Division Multiplexing) method that is adopted in the terrestrial digital broadcasting in Japan and the like, and specifically relates to a technology for automatically detecting a transmission mode.
(2) Description of the Related Art
At present, the OFDM method, which is one of digital modulation methods, is adopted as a transmission method in the terrestrial digital broadcasting in Japan (ISDB-T: Integrated Services Digital Broadcasting-Terrestrial transmission—see Non-Patent Document 1) and in Europe (DVD-T: Digital Video Broadcasting-Terrestrial transmission). The OPDM method is a transmission method suited for high-speed data communications since it is excellent in use of frequency.
Also, in the OFDM method, a redundant signal called guard interval signal is added to prevent communication failures such as a multipath that occur in the actual communication systems.
The transmission symbol of the OFDM signal is composed of an effective symbol and a guard interval. In the transmission method for the terrestrial digital broadcasting in Japan, the guard interval precedes the effective symbol, and has a copy of the latter part of the effective symbol.
With such data structure, if the delay time of the reflected wave is within the range of the guard interval, it is possible to demodulate the effective symbol completely by performing a certain process.
However, since the guard interval is a copy of part of the effective symbol that is the main body of the transmission signal, the transmission efficiency is reduced.
For this reason, for a signal transmission, an optimum transmission mode is selected among a plurality of available transmission modes, taking factors such as the amount of multipath delay for the expected transmission path and the transmission efficiency into consideration. Here, the available transmission modes correspond one-to-one to a plurality of transmission symbol periods, that is to say, combinations of an effective symbol period and a guard interval period.
An OFDM demodulation apparatus for receiving a signal that is transmitted in such a manner is required to recognize the transmission mode in which the signal is transmitted, to correctly demodulate the effective symbol of the received signal.
The OFDM demodulation apparatus can recognize the transmission mode if the transmission mode is determined in advance. However, considering the convenience, it is desirable that the demodulation apparatus can deal with a variety of transmission modes that vary by region or time.
Such a demodulation apparatus is required to have a function to detect the transmission mode of a received signal automatically at a high speed.
The following two are conventionally proposed technologies for automatically detecting the transmission mode.
One proposes a serial type detection method in which all the available transmission modes are checked one by one for the received signal. The other proposes a parallel type detection method for which a plurality of detection circuits corresponding to the available transmission modes are provided in parallel (see Patent Document 1).
The serial type detection method has an advantage that the circuit size is small, but has a defect that it requires a long time to detect the transmission mode. The parallel type detection method has a defect that the circuit size is large, but has an advantage that it requires a short time to detect the transmission mode.
[Patent Document 1]: Japanese Patent Publication No. 2863747
[Non-Patent Document 1]: Standard “Transmission Method for Terrestrial Digital Broadcasting” ARIB STD-B31